docs: def _internal2external_func(bounds):
      """
      Make a function which converts between internal (unconstrained) and
      external (constrained) parameters.
      """
      ls = [_internal2external_lambda(b) for b in bounds]
  
      def convert_i2e(xi):
          xe = empty_like(xi)
          xe[:] = [l(p) for l, p in zip(ls, xi)]
          return xe
  
      return convert_i2e